In order to put energy of sunlight to practical use, a solar cell for converting light energy to electrical energy has been broadly used.
Estimation of the performance of the solar cell as described above has required measurement of an output characteristic as shown in FIG. 14, that is, a current-voltage (I-V) characteristic.
The measurement of the output characteristic of a solar cell is performed by irradiating the solar cell with pseudo sunlight or natural sunlight, varying the operation point of the solar cell between a voltage point Voc under an open state of the solar cell and a current point Isc under a short-circuit state in FIG. 14 and measuring an output voltage and an output current at that time. For example, a capacitance load system as disclosed in Patent Document 1 or an electronic load system as disclosed in Patent Document 2 is known as a system for varying the operation point of the solar cell.
FIG. 15 is a diagram showing the construction of a solar cell characteristic measuring device based on a conventional capacitance load system.
In this measuring device, 1 represents a solar cell as an output characteristic measuring target, 2 represents a voltage detector for detecting an output voltage of the solar cell 1, 3 represents a current detector for detecting an output current of the solar cell, 5 represents a load capacitor which is connected to the output of the solar cell 1 through the current detector 2 and a measuring switch 4 in series, and 6 represents a discharging switch 6 connected to the load capacitor 5 in parallel.
The measurement of the output characteristic is performed by turning on the measuring switch 4 to supply charging current from the solar cell 1 to the load capacitor 5 under the state that the solar cell 1 is irradiated with pseudo sunlight or natural sunlight, thereby applying a load state to the solar cell 1 while the load state varies from a load short-circuit state to a load open state, and measuring an output voltage V and an output current I with the voltage detector 2 and the current detector 3. Re-measurement is executed after charged charges in the load capacitor 5 are discharged by turning off the measuring switch 4 and turning on the discharging switch 6.
Furthermore, as shown in FIG. 16, the measuring device based on the electronic load system is configured so that an electronic load 7 constructed by an electronic control element 71 comprising an electric field effect transistor or the like, an operational amplifier 72 for driving the electronic control element 71, etc. is used in place of the load capacitor 5 and the load current or voltage of the solar cell 1 is adjusted by this electronic load 7. A signal for continuously varying the output current of the solar cell is supplied to an input of the operational amplifier 72 of the electronic load 7 to vary the load state of the solar cell from the load open state to the load short-circuit state as in the case of the capacitor load system, and the output voltage and the output current are measured by the voltage detector 2 and the current detector 3, thereby measuring the output characteristic.
A solar cell has a PN junction portion irrespective of the form of the solar cell such as crystalline form or noncrystalline form, and junction capacitance exists at this junction portion. When load current is taken out from the solar cell, during the period from the time when a load is put in till the time when the value of an output voltage V is stabilized to a true value thereof, this junction capacitance causes existence of a voltage unstable period (t1-t3) which corresponds to the charging/discharging time of charges in the junction capacitance and in which an output voltage V is unstable as shown in FIG. 17.
Therefore, with respect to the conventional output characteristic measuring device for the solar cell, a time delay occurs at the measurement start time until the voltage rises up to the true value thereof, and thus the output voltage cannot be accurately measured just after the load of the solar cell is put in, so that a measurement error occurs. Furthermore, the output current has a larger value because charges are charged/discharged to/from the junction capacitance, and this causes a problem that the output characteristic cannot be accurately measured.